The present invention relates to a method and apparatus for providing a predistortion function in a power amplifier.
In the field of radio communication systems, it is a well-known problem that the power amplifiers present in transmission equipment operate in a non-linear fashion when the power amplifiers are operated near their peak output. As a result, the power amplifier introduces significant signal distortion that can appear in various forms. For example, if more than one signal is input into the power amplifier or power amplification stage, its non-linear characteristics can cause an undesirable multiplicative interaction of the signals being amplified, and the power amplifier""s output can contain intermodulation products. These intermodulation products cause interference and crosstalk over the power amplifier""s operational frequency range.
In power amplifier design, there is a trade off between distortion performance and efficiency. Linear amplifiers that operate under xe2x80x9cClass Axe2x80x9d conditions create little distortion but are inefficient, whereas nonlinear amplifiers operated under xe2x80x9cClass Cxe2x80x9d conditions are reasonably efficient but introduce significant distortions. While both efficiency and distortion are important considerations in amplifier design, efficiency becomes increasingly important at high power levels. Because of their efficiency, nonlinear amplifiers are largely preferred, leaving the user with the problem of distortion.
In order to employ nonlinear power amplifiers, techniques have been used to improve linearity and thereby reduce the effects of interference and crosstalk. Linearity can be achieved by application of various linearization techniques that reduce the distortion caused by nonlinear amplification. Conventional amplifier linearization techniques can be broadly categorized as feedback, feedforward, or predistortion.
The last mentioned technique, predistortion, intentionally distorts the signal before the power amplifier so that the non-linearity of the power amplifier can be compensated. According to this technique, linearization is achieved by distorting an input signal according to a predistortion function in a manner that is inverse to the amplifier characteristic function. The predistortion technique can be applied at radio frequency (RF), intermediate frequency (IF), or at baseband.
In the baseband domain, the input signal information is at a much lower frequency, allowing digital methods to be employed. The predistortion function is applied to the input signal with the resulting predistorted signal being upconverted to IF and then finally to the RF carrier frequency. It is also possible to apply adaptive predistortion techniques where feedback from the output of the amplifier is used to update and correct the predistortion function.
The form of the predistortion function is dependent upon the model used to characterize the output of the amplifier. Predistortion functions in the baseband domain are typically implemented as a table of gain and phase weighting values within a digital signal processor. A Cartesian feedback method employs a quadrature representation of the signal being amplified. The incoming quadrature signals I and Q are compared to the feedback quadrature signals. Thus, there are two sets of coefficients, one for each quadrature channel, that are being updated to model the predistortion characteristics. In this manner, gain and phase non-linearities within the amplifier can be compensated. Performance is dependent on the size of the look up table and the number of bits used to represent the signal. Better performance and more adaptivity is achieved with larger look up tables and more bits albeit at the expense of longer processing times.
Predistortion functions are also modeled as polynomials. Here, the complex polynomial must be able to characterize the inverse of the amplifier, which may not analytically exist and which must then be approximated. In order to accurately estimate the inverse, the polynomial requires high order terms, with associated quantization errors and a less accurate polynomial fit.
Adaptive methods generally process and model current amplifier characteristics. The current output signal of the amplifier is contrasted against the current input signal to the amplifier. Past inputs are not considered. However, amplifier characteristics are dependent upon frequency due to the speed in which input signals change amplitude as a function of frequency. Exclusion of past inputs precludes modeling those changes and limits the accuracy with which the amplifier can be modeled, thereby limiting the bandwidth.
Accordingly, there is a need for a power amplifier predistortion system that can quickly and efficiently obtain an optimum predistortion function for frequency dependent amplifiers.
The present invention teaches an apparatus and method for calculating the predistortion function from a power amplifier. A predistortion module generates a predistorted signal in response to an input signal and a predistortion function. The predistortion function calculating module generates a predistortion function in response to given amplifier characteristics. To more readily calculate the predistortion function, the predistortion function calculating module uses a magnitude of the input signal at a given time as an estimate of the predistorted signal at that given time to calculate the predistortion function. For example, in calculating the inverse of an amplifier characteristic curve, the magnitude of the current signal sample can be a function of the magnitude of the current predistorted signal. By replacing the magnitude of the current predistorted signal sample with the magnitude of the current input signal sample, the predistorted signal can determined.
In an exemplary embodiment of the present device, the predistortion function calculating module, in calculating the predistortion function, estimates the magnitude of the predistorted signal on an iterative basis.
In another exemplary embodiment of the present device, time spaced samples of the predistorted signal are processed by an amplifier with memory, and corresponding time indexed coefficients are used in characterizing the amplifier and in the calculation of the predistortion function. Each time spaced sample of the predistorted signal is generated from a corresponding time spaced instance of the input signal.
Importantly, power amplifiers have memory and their characteristics depend upon signal frequencies. By including prior instances of the input signal, the present invention can model the impact of both current and past instances of the input signal and therefore model non-linear and frequency dependent aspects of the power amplifier.
By incorporating the frequency dependency of the power amplifier, the present invention allows efficient power amplifier operation at wider bandwidths where sensitivity to frequency is more problematic. The invention further enables power amplifiers to be operated in the nonlinear region near saturation, yet suppresses undesirable intermodulation products. Resort to a larger amplifier, to keep operation within the linear region, is avoided. Power amplifier sizes are kept small with associated cost savings, particularly important in the field of wireless communications.
The above factors make the present invention essential for effective power amplifier predistortion.